South Jersey Skies: The most difficult mission

In many ways, the recent visit to the Hubble Space Telescope by the Space Shuttle was the most difficult servicing mission of the four.

Keith Johnson

For just one example, consider the Space Telescope Imaging Spectrograph (referred to in typical NASA acronym-speak as the STIS, and pronounced, believe it or not, “stis”).

The STIS has a low-voltage power supply. Like most Hubble equipment, it employs backup components. When one component fails, the backup goes into service.

But the Hubble has been in space for nearly 20 years. Two components of the STIS power supply, the primary and the backup power converters, were both dead by August 2004. The instrument is immensely valuable, so mission planners decided to try replacing one of the converters during the servicing mission, even though the instrument had been designed to be serviceable only on Earth.

It was a challenge in many ways. Access to the STIS required removal of a cover plate. This plate was attached to the instrument by 111 tiny specialized screws (usually referred to as fasteners.) The area had not been designed for in-orbit access, and it wasn’t even very easy for astronauts to get to.

There were three different sizes of fasteners. Further, the fasteners had been in the harsh environment of space for nearly two decades. Some of them could have frozen in place. If so, and if an eager astronaut used too much force to break a stubborn one free, the fastener could break, making it irremovable.

Finally, there was the question of what to do with the fasteners once they were taken out. A wayward fastener, floating randomly through the telescope, could damage optical components or cause short circuits. All 111 tiny fasteners had to be safely captured, not an easy job while wearing bulky gloves and working without gravity.

The solution came in two parts. To ensure safe removal of the fasteners, a new tool called the Mini-Power Tool (or “MPT”) was created. The MPT is essentially a pistol-grip multipurpose electric screwdriver/drill. It is small, high-speed, but low-torque. Torque, or twisting power, was set low to avoid breaking a fastener. Drill speed was set high to allow the spacewalkers to get the job done in as short a time as possible.

The second part of the solution was the Fastener Capture Plate (you can probably guess the acronym.) This is a transparent plastic plate that fastened over the STIS cover plate. The FCP had small machined holes precisely aligned with each fastener. With openings large enough for the drill bits but smaller than the fasteners being removed, this ingenious device trapped the fasteners and prevented them from drifting away. This means that the drill bits themselves had to be quite small, since the holes through which they had to pass were small.

In addition, each hole was color-coded to match one of the three drill bits, one for each size of fastener. Fortunately, none of the astronauts was color-blind.

During EVA ( “Extra-Vehicular Activity”) number 4, astronaut Mike Massimino, wearing his bulky EVA suit with its pressurized, barely-flexible gloves, had to fasten the FCP in place over the STIS access plate (just saying the acronyms makes me feel like I’m working at NASA!) Then he inserted the proper bit into the MPT, poked the tiny bit through the proper tiny hole, and gingerly activated the screwdriver, trying to be careful not to break a fastener -- and all while not being able to really see what was going on all that well!

Moreover, he had to do this successfully 111 times, then remove and store the FCP, insert and connect the new components and clean up, while racing the clock to complete a spacewalk of more than five hours before his oxygen and battery power gave out.

With a nod to sensibility, NASA at least replaced the multiple-screw arrangement with three simple latches!

It’s a tribute to both the tool designers and the astronauts that several difficult tasks like this one were accomplished successfully in a five-day time window.

The telescope will be subjected to a series of checkouts during the next three or four months, and we should be seeing new pictures in August or September. Keep an eye on “Astronomy Picture of the Day” (http://antwrp.gsfc.nasa.gov/apod/astropix.html) at that time for the first one!

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Keith Johnson is the director of the Edelman Planetarium at Rowan University. The planetarium is closed for the summer, but will reopen to the public in the fall. Information is available at www.rowan.edu/planetarium/.